After meeting with lawmakers on Capitol Hill, the Facebook chief executive was interviewed onstage for about an hour at the Newseum by James Bennet, editor in chief of The Atlantic.

"The purpose of this trip is largely for immigration and not for Facebook," Zuckerberg said.

With the launch of its political advocacy group Fwd.us, Zuckerberg has assumed a much larger role in national politics. His top priority has been immigration reform, and on Wednesday he met with House Speaker John Boehner and Democratic Sen. Charles E. Schumer, one of the Gang of Eight who crafted the Senate bill on immigration reform. On Thursday he will meet with House and Senate leadership including House Minority Leader Nancy Pelosi (D-San Francisco).

Even as hope for passing legislation in the House has continued to dim, Zuckerberg said he remains "optimistic."

He used Silicon Valley terminology when he said Fwd.us needed to "debug" some things. The group spurred an early backlash when it sponsored ads that criticized President Obama's healthcare policy and promoted the Keystone XL pipeline.

But Zuckerberg remained focused on the broader issue of immigration reform.

"Actually addressing the 11 million undocumented folks is a lot bigger problem than high-skilled workers," he said.

Fwd.us president and Zuckerberg friend Joe Green was in the audience.

"He makes the case for fixing our broken immigration system as well as anybody," Green said before the event.

As he did a week ago during the TechCrunch Disrupt conference in San Francisco, Zuckerberg called for more transparency from the National Security Agency on requests for Facebook users' information.

He said the "trust metrics" for technology companies plummeted after the revelations that technology companies hand over user information to intelligence agencies.

"I don't think the answer is no governmental requests for national security," Zuckerberg said.

But, the more transparent the government is, "the more comfortable the public will be," he said.

Even though he was in a town that is sharply divided along party lines, Zuckerberg declined to divulge his party affiliation.

"It's hard to affiliate as being a Democrat or Republican," he said. "I'm pro-knowledge economy."